USING THE SCHMIDT HAMMER TO IMPROVE THE FORECASTING ACCURACY OF THE ROCKFALL ACTIVITY INDEX (RAI)

Abstract

The Schmidt hammer is a widely used and inexpensive instrument for estimating rock strength either in the lab or in the field. Our research team tested the accuracy and repeatability of the Schmidt hammer to estimate rock strength on six Alaskan rock slopes and four Washington/Oregon rock slopes. We determined in situ rock hardness by using two different Schmidt hammers (Types L and N); conducted unconfined compressive strength (UCS) testing for select Alaskan rock samples; and summarized the advantages and disadvantages of using the Schmidt hammer in the field. Parameters that potentially affect Schmidt hammer results include testing methodology, sample testing conditions, and data reduction. Our results indicated that major structures within a rock unit (such as bedding or foliation), variation in mineralogy, and moisture content will significantly affect Schmidt hammer results. After data collection, several correction methods can be used to process the Schmidt hammer results. The choice of method depends on the intent of the measurements (i.e., strength of the intact rock or the rock mass), and the application of any method can alter the final results. Our UCS results generally correlated to the Schmidt hammer rebound values (e.g., rock types with high rebound values also had high UCS values). Before utilizing the Schmidt hammer, we suggest users determine the final goal before selecting the Schmidt hammer and testing methodology; identify the rock type and potential discontinuities that can influence results; identify the bias in sample or site selection; and select the most applicable data reduction method for the identified goal.